Disease, advancing age, and trauma can lead to changes in various bones, discs, joints, and ligaments of the body. Some changes manifest themselves in the form of damage or degeneration to a spinal disc. It is known that an intervertebral disc can be subject to damage, such as compression, deformation, displacement, or wear, and more generally degeneration associated with the mechanical stresses that are applied thereto and that lead to anatomical and functional destruction of the disc and of the vertebral segment. This damage to the disc alters its mechanical behavior and causes instability which can, in turn, give rise to a painful osteoarthritic reaction.
This pain can sometimes be eliminated by spinal fusion in which two adjacent vertebral bodies are joined together after removing the intervening intervertebral disc. A prosthetic device is usually placed between the two adjacent vertebral bodies, in place of the removed disc, to fill the space left by the removed disc and to allow bone to grow between the two vertebral bodies.
Alternatively, proposals have been made to replace the defective disc with an artificial disc that preserves the natural mobility between adjacent vertebral bodies. For example, such prostheses can include first and second plates for fixing to adjacent vertebral bodies and an articulating mechanism, such as a ball joint, interposed between the two plates. Although such prostheses makes it possible to restore an appropriate height to the intersomatic gap, the articulating mechanism of the prosthesis can be subject to high levels of friction, thus making it sensitive to wear.
Other artificial discs include ceramic bearing surfaces positioned within titanium endplates. While the ceramic materials reduce wear, these implants are limited by small bearing surfaces. In addition, mounting stiff ceramic inserts into titanium endplates can generate high stresses in the ceramic material and cause disc failure. Thick titanium endplates can also interfere with post-operative imaging, an important diagnostic tool.
Despite known prostheses for the replacement of discs, there remains a need for additional prostheses that are sufficiently wear resistant, compatible with post-operative imaging, and suitable for secure and effective implantation within a patient's spine.